There are many metal-glass-ceramic applications and systems which have in common the bonding of a glass or ceramic material to the surface of a metal. One common application is the preparation of hermetic seals for metal semi-conductor devices. Characteristically, in such applications, the glass or ceramic does not bond directly to the metal, but to an oxide film which is conventionally formed by a thermal treatment of the metal prior to bonding. The resulting oxide acts as the bonding agent in that it is bonded to the underlying metal and the glass or ceramic material. Therefore, the characteristics of the metal oxide exert a profound influence upon the overall properties of the glass or ceramic-to-metal bond.
Most metal oxides and mixtures of metal oxides which have been developed upon the surface of most metals and alloys, in accordance with the prior art, possess poor mechanical properties. Therefore, the prior art glass or ceramic-to-metal composites and seals have been designed to minimize the stresses developed at the bond interface because of its relatively poor strength. It is generally known that those glasses and ceramics which possess the desirable bonding and sealing characteristics also have coefficients of thermal expansion which are substantially smaller than the coefficients of thermal expansion of most metals and alloys. Therefore, the prior art has developed a series of low expansivity metal alloys, which have coefficients of thermal expansion for a limited temperature range which reasonably closely match the coefficients of thermal expansion of many sealing glasses or ceramics as set forth in Table I.
TABLE 1 __________________________________________________________________________ Materials Composition Thermal Expansion Coeff. in/in/.degree. C. __________________________________________________________________________ A. METALS KOVAR ASTM No. F-15-68 Fe + 29% Ni + 17% Co + 0.45% Mn + 0.10% 41 .times. 10.sup.-7 (at 20.degree. C.) (RODAR) + 0.02% C. 60 .times. 10.sup.-7 (at 100.degree. C.) 118 .times. 10.sup.-7 (at 500.degree. C.) 49 .times. 10.sup.-7 (range 0.degree.-400.degree. C.) NICKEL 100% Ni 128 .times. 10.sup.-7 (at 20.degree. C.) NIRON 52 ASTM No. F-30-68 51% Ni, 49% Fe 98 .times. 10.sup.-7 (range 25.degree.-500.degree. C.) NIRON 42 ASTM No. F-30-68 41% Ni, 59% Fe 47 .times. 10.sup.-7 (range 30.degree.-300.degree. C.) NIRON 46 ASTM No. F-30-68 46% Ni, 54% Fe 77 .times. 10.sup.-7 (range 30.degree.-350.degree. C.) DUMET ASTM No. F-29-68 43% Ni, 57% Fe 68 .times. 10.sup.-7 (range 30.degree.-400.degree. C.) SYLVANIA No. 4 42% Ni, 6% Cr, 52% Fe 89 .times. 10.sup.-7 (range 30.degree. -350.degree. C.) ASTM No. F-31-68 B. GLASSES AND CERAMICS SODA - LIME - SILICA GLASS 70% Si O.sub.2, 11% Ca O, 14% Na.sub.2 O, + Al.sub.2 O.sub.3 + Mg O 90 .times. 10.sup.-7 (range 0.degree.-100.degree. C.) PORCELAIN (electrical 40% Leucite (K.sub.2 O, Al.sub.2 O.sub.3, 4 Si O.sub.2) 60 .times. 10.sup.-7 (range 0.degree.-1000.degree. C.) granulation 30% Mullite (3Al.sub.2 O.sub.3, 2 Si O.sub.2) 30% Si O.sub.2 SEALING GLASS TYPE 101 56% Si O.sub.2, 1.5% Al.sub.2 O.sub.3, 4.0% K.sub.2 O, 29.0% PbO 92 .times. 10.sup.-7 (range 30.degree.-300.degree. C.) ASTM No. F-79-67T __________________________________________________________________________
Unfortunately, the range of low expansivity alloys which have been developed are not otherwise particularly desirable materials. Initially, these alloys contain nickel as a major alloying addition, and are thus quite costly and exhibit relatively poor corrosion and oxidation resistance.
The above alloys often contain other additions such as chromium and cobalt. The thermal oxide film which is formed on these alloys is generally of mixed composition which is dependent upon the actual alloying additions present in the metal. It is known that to obtain good glass adherence particularly to the above alloys possessing bare surfaces, a separate thermal pre-treatment must often be performed in a special atmosphere such as wet hydrogen to form an oxide film possessing the required properties. The treatment is time consuming and produces an oxide generally quite thick in nature and characterized by poor mechanical properties.